New tires = lower MPG
 

I was doing some reading on rolling resistance, and found out that new tires will typically decrease MPG because of increased RR (and the fact that the odometer is turning slower due to larger OD). Pretty interesting articles over at TireRack.

https://www.tirerack.com/tires/tirete...sp?techid=177& (You might want to start with parts 1 and 2 first)

-BC

Yup, I've got that info buried somewhere in the huge unwieldy piles of data and hot air in one of my tire threads. Sometimes I mention it in other people's tire threads; I tell them that any new tire is going to have more RR than their worn tires.

The odometer turning slower due to larger OD would affect the accuracy of your FE measurement (making it read slightly less than it really is), but its true effect would probably be a minuscule increase in real FE.

Hey, come to think of it, I wonder if my severely worn tires are part of why my FE seems a little better than it used to be. I'm challenging myself to see how long I can go with them before I put on the replacements that I already have...

I replaced my last set of tires before they were worn down to the point that they would not pass the state safety inspection. I prob ably could have got another 5,000 to 10,000miles out of them before the tread wear indicators popped up, but a fantastic deal came my way on the tires I wanted, so I replaced them.

Makes you want to drive around on may pops doesn't it.

Lots of great info about tire tech from a tire engineer from NC.

https://www.barrystiretech.com/index.html

That site gets my endorsement, for whatever that's worth. I've liked it for a long time. I find it very informative and there is very little I disagree with there.

I've used 175/70-13 size tires on my '92 Civic VX for years due to lack of availability of quality OEM size 165/70-13s. Any idea how much the larger diameter tires affect my mpg figures? Measured trip odometer readings show I get a consistent 40-45 mpg. Am I doing better or worse than that in actual numbers?

Assuming that your car has not been adjusted for the new tires, you are getting better fuel economy than your calculations. You can compensate by calculating in an extra 2.5%.